1. Field of the Invention
This invention relates to equipment, transponders and methods for optical fiber transmission. Specifically, it relates to a WDM (Wavelength Division Multiplexing) optical fiber transmission device and wavelength division multiplexing system. More specifically, it relates to a wavelength division multiplexing optical fiber transmission system which multiplexes plural optical signals of different wavelength in an optical fiber to transmit information, and to a transponder device or a wavelength division multiplexing optical fiber transmission device using this system.
2. Description of the Related Art
Wavelength division multiplexing (WDM) is an extremely useful technique for increasing the volume of optical fiber communications. A typical example of the construction of a prior art wavelength division multiplexing optical transmission system comprises a wavelength division multiplexing optical transmission device 152 (transmitting side) and a wavelength division multiplexing optical transmission device 153 (receiving side) disposed at sites 141-1, 141-2 located at two points separated by a distance of several kilometers to several thousand kilometers, as shown in FIG. 5. The two devices are connected by inter-site optical fiber circuits 144-1, 144-2, and a wavelength division multiplexing optical repeater 151. The figure shows only transmission from the site 141-1 to the site 141-2, although this is generally combined with a wavelength division multiplexing optical transmission device with reverse direction.
A SONET (Synchronous Optical NETwork)/SDH (Synchronous Digital Hierarchy) terminals, ADM devices, or prior art information communications devices 150-1, 150-2 which perform information communications, such as IP routers, are provided in the transmitting side site 141-1, and optical signals are transmitted to the wavelength division multiplexing transmission device 152 via intra-site optical fiber circuits 142-1-142-n. The intra-site optical fiber circuits 142 have a distance of the order of several m—several tens of kilometers, and uses for example a SONET/SDH signal format such as OC-12 (600 Mbit/s) or OC-48 (2.5 Gbit/s).
From the viewpoint of cost, IM/DD (Intensity Modulation/Direct Detection) of a laser diode operating in, for example, the 1.3 μm wavelength band is used for the optical transceivers in the intra-site optical fiber circuits 142. As the signals transmitted in the intra-site optical fiber circuits have problems of wavelength band, wavelength interval, spectral purity, wavelength accuracy and dispersion tolerance, they are unsuitable for wavelength division multiplexing transmission over long distance optical fibers. Therefore, the signals are converted to plural different wavelengths (λ1-λn) for inter-site wavelength division multiplexed transmission by the transmitting side transponder devices 120-1-120-n, wavelength division-multiplexed by an optical multiplexer 145, and then output to the inter-site optical fiber circuit 144-1.
The wavelength division-multiplexed optical signals transmitted to the inter-site optical fiber circuit 144-1 are relay amplified by the optical repeater 151, transmitted along the inter-site optical fiber circuit 144-2, input to a wavelength division multiplexing optical transmission device 153 on the receiving side, wavelength-demultiplexed into optical signals of wavelength λ1-λn by an optical demultiplexer 146, and then respectively input to receiving side transponder devices 130-1-n.
In the prior art transmission side transponder device 120, an optical signal of wavelength λa is received by an intra-site transmission optical receiver 123 from the intra-site optical fiber 142, and the optical signal is then converted to an inter-site transmission signal format as a wavelength division-multiplexed optical signal (wavelength λ1) by an inter-site transmission optical transmitter 124, and output, as shown in FIG. 6.
The wavelength of the inter-site transmission optical signal is normally in the 1.5 μm band which is suitable for amplification by an optical fiber amplifier, and generally coincides with wavelength grids (50 Ghz, 100 GHz, 200 GHz interval) which are wavelengths standardized for use with WDM. Also, as the inter-site transmission distance may attain several 100—several 1000 km, external optical modulation, which is suitable for long distance transmission, is often used. For the inter-site transmission signal format, a SONET/SDH format basically identical to that used for the intra-site transmission format is widely adopted, but in recent years, a wavelength wrapper with the addition of bit error correction between transponder sections or a monitoring function has also been considered.
In the receiving side transponder device 130, an optical signal input from an inter-site side input fiber 131 is received by an inter-site transmission optical receiver 133, the optical signal is converted to an intra-site circuit signal format or the wavelength λa by an intra-site transmission optical transmitter 134, and output to an intra-site side output optical fiber 132, as shown in FIG. 7. These optical fibers are respectively connected to intra-site information communications devices 150-3, 150-4 via intra-site optical fiber circuits 143-1-143-n in FIG. 5.
In the prior art wavelength multiplexing transmission system, the transmission speeds on the intra-site side and inter-site side are identical in principle. For example, in the case where the transmission speed of the intra-site optical fiber circuits 142, 143 shown in FIG. 3 is 2.5 Gbyte/s, the transmission speed of an optical signal of wavelength λn in the inter-site optical fiber circuit 144 was also 2.5 Gbyte/s. In this prior art device, if the information transmission amount between the information communications devices 150-2, 150-4 increases and the bit rate is increased, for example, transmission in the inter-site transmission part is difficult. As the maximum transmission distance of optical fiber transmission is inversely proportional to the square of the bit rate, when the transmission speed is as high as 10 Gbit/s or 40 Gbit/s, the maximum transmission distance of the inter-site transmission part very rapidly becomes shorter. For example, the maximum transmission distance at 2.5 Gbit/s is 600-1200 km, but as it is of the order of several tens of km at 10 Gbit/s and no more than several km at 40 Gbit/s, it is difficult to achieve long-distance transmission at these speeds. If a dispersion compensation technique or dispersion shifted fiber (DSF) is used, this value can be improved to some extent, but even then, the usual limit of the transmission distance is around 500 km at 10 Gbit/s and 40 km at 40 Gbit/s. Therefore, if the transmission speed of the intra-site optical fiber circuits 142, 143 is increased, the number of repeaters in the inter-site part must be increased, and the cost increases.
To deal with this problem, the system of FIG. 5 shows an example where the number of inter-site circuits in the information communications device 150-1 is increased to, for example, three. In this example, even if the transmission amount is increased by three times, the transmission speed in the inter-site transmission part remains the same, so the above problem can be avoided. However, the number of intrasite circuits and the number of inter-site transceivers also increases by three times, so there are other problems in that cost and intra-site circuit management difficulties increase.
Further, due to the aforesaid transmission distance problem, the intra-site transmission speed of the wavelength division multiplexing transmission device 152 is normally limited in practice to a value such as 2.5 Gbit/s-10 Gbit/s, and the speed cannot be increased without limit. Therefore, even if a faster intra-site circuit interface such as 10 Gbit/s or 40 Gbit/s were developed, it could not be applied to this wavelength division multiplexing transmission device.